It's 2lb difference. If you're finding it "tiring" right now to carry your laptop plus all your stuff then you're probably carrying a bag which weighs a lot more than just the laptop.
Shaving such a small amount off the laptop's weight isn't really going to make a difference to you - you'll still be carrying all those books plus any accessories for the laptop.
Agreed. A 13 lb backpack isn't going to be much easier to carry than a 15 lb backpack, but then again, I have muscles. I don't have a lot of muscle, but the stuff I have is there when I need them. That, and spending $1800 in order to shave 2 lbs would be a complete ripoff.
Now if you were talking about a netbook, that'd be different. Physically, it's a lot smaller, and would perhaps allow you to use a smaller bag.